Abstract The sustainability of groundwater resources for agricultural and domestic use is dependent on both the groundwater recharge rate and the groundwater quality. The main purpose of this study was to improve the understanding of the timing, or seasonality, of groundwater recharge through the use of stable isotopes. Based on 768 groundwater samples collected from aquifers underlying natural resources districts in Nebraska, the isotopic composition of groundwater (δ 2 H and δ 18 O) was compared with that of precipitation by (a) mapping the isotopic composition of groundwater samples and (b) mapping a seasonality index for groundwater. Results suggest that for the majority of the state, groundwater recharge has a nongrowing season signature (October–April). However, the isotopic composition of groundwater suggests that in some intensively irrigated areas, human intervention in the water cycle has shifted the recharge signature towards the growing season. In other areas, a different human intervention (diversion of Platte River water for irrigation) has likely produced an apparent but possibly misleading nongrowing season recharge signal because the Platte River water differs isotopically from local precipitation. These results highlight the need for local information even when interpreting isotopic data over larger regions. Understanding the seasonality of recharge can provide insight into the optimal times to apply fertilizer, specifically in highly conductive soils with high leaching potential. In areas with high groundwater nitrate concentrations, this information is valuable for protecting the groundwater from further degradation. Although previous studies have framed nongrowing season recharge within the context of future climate change, this study also illustrates the importance of understanding how historical human intervention in the water cycle has affected groundwater recharge seasonality and subsequent implications for groundwater recharge and quality.
In the last decades, human activity has been contributing to climate change that is closely associated with an increase in temperatures, increase in evaporation, intensification of extreme dry and wet rainfall events, and widespread melting of snow and ice. Understanding the intricate linkage between climate warming and the hydrological cycle is crucial for sustainable management of groundwater resources, especially in a vulnerable continent like Africa. This study investigates the relationship between climate-change drivers and potential groundwater recharge (PGR) patterns across Africa for a long-term record (1960-2010). Water-balance components were simulated by using the PCR-GLOBWB model and were reproduced in both gridded maps and latitudinal trends that vary in space with minima on the Tropics and maxima around the Equator. Statistical correlations between temperature, storm occurrences, drought, and PGR were examined in six climatic regions of Africa. Surprisingly, different effects of climate-change controls on PGR were detected as a function of latitude in the last three decades (1980-2010). Temporal trends observed in the Northern Hemisphere of Africa reveal that the increase in temperature is significantly correlated to the decline of PGR, especially in the Northern Equatorial Africa. The climate indicators considered in this study were unable to explain the alarming negative trend of PGR observed in the Sahelian region, even though the Standardized Precipitation-Evapotranspiration Index (SPEI) values report a 15% drought stress. On the other hand, increases in temperature have not been detected in the Southern Hemisphere of Africa, where increasing frequency of storm occurrences determine a rise of PGR, particularly in southern Africa. Time analysis highlights a strong seasonality effect while PGR is in-phase with rainfall patterns in the summer (Northern Hemisphere) and winter (Southern Hemisphere) and out-of-phase during the fall season. This study helps to elucidate the mechanism of the processes influencing groundwater resources in six climatic zones of Africa, even though modeling results need to be validated more extensively with direct measurements in future studies.
Agroecosystems impact water resources by consuming most fresh water through irrigation and by changing water partitioning at the land surface. The study assesses impacts of agroecosystems on groundwater resources in the Texas Central High Plains (37,000 km2 area) by evaluating temporal variations in groundwater storage and quality. Percolation/recharge rates were estimated using groundwater Cl data and using unsaturated zone matric potential and water-extractable chloride and nitrate from 33 boreholes beneath different agroecosystems. Total groundwater storage decreased by 57 km 3
Although irrigated agriculture is the primary consumer of global groundwater resources, information on recharge rates and sustainable irrigation is limited. The study objective was to fingerprint irrigation return flow to quantify percolation/recharge and to estimate sustainable irrigation levels. This paper focuses on water quantity; a companion paper addresses water quality. Soil samples from 13 boreholes drilled beneath irrigated agroecosystems in the southern High Plains were analyzed for matric potential and water‐extractable Cl and NO 3 . Unsaturated zone pore water beneath irrigated agroecosystems can be fingerprinted by higher matric potentials (wetter soils, median mp: −40 m) and higher NO 3 ‐N (median 71 mg/L) than beneath natural ecosystems (mp −200 m; NO 3 ‐N 8.1 mg/L) and by higher Cl (720 mg/L) than beneath rain‐fed agroecosystems (8.4 mg/L). The range in percolation/recharge rates beneath irrigated agroecosystems is 18–97 mm/a (median 41 mm/a; 5% of irrigation + precipitation) and occurs primarily in response to extreme precipitation events. Similarity in percolation/recharge rates beneath irrigated and rain‐fed (4.8–92 mm/a) agroecosystems was unexpected and is attributed to low irrigation applications (median 300 mm/a) and increased crop yield and evapotranspiration in irrigated areas. Regional water table declines are unsustainably large (≥ 30 m over 10,000 km 2 ) in the north and are much lower in the south. Sustainable irrigation in the south would require reduction of the irrigated area from 23% to 9%. Methods developed for quantifying recharge and sustainable irrigation application rates can be applied to groundwater‐fed irrigated areas in semiarid regions globally.
Unsaturated zone salt reservoirs are potentially mobilized by increased groundwater recharge as semiarid lands are cultivated. This study explores the amounts of pore water sulfate and fluoride relative to chloride in unsaturated zone profiles, evaluates their sources, estimates mobilization due to past land use change, and assesses the impacts on groundwater quality. Inventories of water‐extractable chloride, sulfate, and fluoride were determined from borehole samples of soils and sediments collected beneath natural ecosystems ( N = 4), nonirrigated (“rain‐fed”) croplands ( N = 18), and irrigated croplands ( N = 6) in the southwestern United States and in the Murray Basin, Australia. Natural ecosystems contain generally large sulfate inventories (7800–120,000 kg/ha) and lower fluoride inventories (630–3900 kg/ha) relative to chloride inventories (6600–41,000 kg/ha). Order‐of‐magnitude higher chloride concentrations in precipitation and generally longer accumulation times result in much larger chloride inventories in the Murray Basin than in the southwestern United States. Atmospheric deposition during the current dry interglacial climatic regime accounts for most of the measured sulfate in both U.S. and Australian regions. Fluoride inventories are greater than can be accounted for by atmospheric deposition in most cases, suggesting that fluoride may accumulate across glacial/interglacial climatic cycles. Chemical modeling indicates that fluorite controls fluoride mobility and suggests that water‐extractable fluoride may include some fluoride from mineral dissolution. Increased groundwater drainage/recharge following land use change readily mobilized chloride. Sulfate displacement fronts matched or lagged chloride fronts by up to 4 m. In contrast, fluoride mobilization was minimal in all regions. Understanding linkages between salt inventories, increased recharge, and groundwater quality is important for quantifying impacts of anthropogenic activities on groundwater quality and is required for remediating salinity problems.
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